Such safety circuits are used, in general, to ensure that people or objects who or which come between two edges moving toward each other and are caught by them (such as between a revolving door and its associated wall or stand body) will not be injured or damaged. The principal task of such an arrangement being to stop the part on which the moving edge is located as soon as the contact- sensitive or pressure-sensitive switching member sends a corresponding interference signal. The motor-driven part can be stopped in two different ways, namely, by switching off the drive motor and simultaneously switching on a braking or arresting device, or by switching on the electronic braking or arresting device. The braking torque that must be applied in the latter case must be high enough so that it cannot be overcome by the drive torque of the motor and the moment of inertia of the moving mass. However, this condition is also important for the case in which the motor drive is not switched off due to any switching disorder.
Since increased safety against the risk of injury is required for people, conventional safety circuits, which are only singly secured, are not sufficient for such cases. This is due to the fact that it can never be ruled out that failure of the unit controlling the switching on of the braking or arresting device can also occur simultaneously with a failure of the unit controlling the switching off of the drive motor.